The present invention relates to a temperature sensor, and more particularly to a temperature sensor for a radiant heating unit that can be used in a cooking stove.
A temperature sensor of this type is described, for example in European patent publication EP 0 141 923 B. Two separately controllable heating coils are arranged in a cup of the radiant heating unit. The cup has a wrap-around projection that extends from the bottom of the cup and separates the two heating coils. The temperature sensor has a tube that extends across both heating coils and a three-segment rod that is supported in the tube. The center section of the rod extends only across the region of the centrally located heating coil. The end faces of the two outer sections of the rod abut the center section of the rod and have a thermal expansion coefficient which corresponds at least to the thermal expansion coefficient of the surrounding tube. The tube is formed as one piece, whereas the thermal expansion coefficient of the center section of the rod is smaller than the thermal expansion coefficient of the surrounding tube.
In a different design, the thermal expansion coefficient of the center section of the rod is greater than the thermal expansion coefficient of the surrounding tube, and the outer sections of the rod have a thermal expansion coefficient that is no larger, and preferably smaller, than the thermal expansion coefficient of the surrounding tube.
With this arrangement, the exterior sections of the rod which are located in the regions of the outer heating coil, expand equally or more than the surrounding tube, which over-compensates the influence of the addible heating coil. The temperature sensor then measures essentially the temperature in the central region of the cup and hence the temperature in the region of the inner heating coil.
European patent publication EP 0141 923 B also describes a solution wherein the tube is likewise formed in three sections, and wherein the sections of the tube have essentially a one-to-one correspondence with the sections of the rod. The center section of the tube has here a greater thermal expansion coefficient than the outer sections of the tube; likewise, the outer sections of the rod have a greater thermal expansion coefficient than the center section of the rod. The outer sections of the tube and the rod, respectively, are arranged in the region of the outer heating coil. In this embodiment, the influence of the addible outer heating coil is also compensated.
With these conventional approaches, although the influence of the outer heating coil is compensated, the switching point of the temperature sensor is still shifted, because heat is necessarily transferred from the cup of the radiant heating unit into the heating space surrounding the radiant heating unit due to thermal conduction and radiation. The switch housing then also expands according to its thermal expansion coefficient and hence affects the switching point.
Conventional temperature sensors for radiant heating units with a continuous rod that is supported in a tube connected with a switch housing and extending into the switch housing also experience a noticeably drop in the switch actuation temperature when the switch housing heats up. Switch housings are mostly formed of steatite and are located outside the cup of the radiant heating unit. The switch actuation temperature therefore must be set high enough so as to sufficiently exceed the actual switch actuation temperature for a cold switch housing.
It would therefore be desirable and advantageous to provide a temperature sensor that does not require a disproportionate increase of the desired switch actuation temperature when the switch housing is cold, which obviates prior art shortcomings and is able to prevent an initial overshoot of the set cooking temperature.